Patent GB-847795-A (GB)
Title: Improvements relating to correcting for transmission variations in television and other signal transmission systems
Inventor: JAMES IVANHOE JOHN PENFOUND
IPC: H04N5/217
Filed: 1955-11-24
Abstract: 847,795. Television. ELECTRIC & MUSICAL INDUSTRIES Ltd. Nov. 15, 1956 [Nov. 24, 1955], No. 33665/55. Drawings to Specification. Class 40(3) [Also in Group XL(c)] Undesired variations in the signals produced by means for translating images into electrical signals (e.g. due to sensitivity variations in pickup tubes) are stored in a magnetic storage device and employed to modify the signal output of the image translating means to compensate for such variation. The invention is concerned with compensating for such variations in individual pick-up tubes, particularly in arrangements where a plurality of tubes is used as in colour television cameras and also in arrangements in which pick-up tubes are employed in co-operation with image reproducing tubes such as in systems for converting from one standard to another (e.g. colour standards, scanning standards, band width compression systems &c.) in which case undesired variations caused by the image reproducing tubes are also compensated. To perform the invention the pick-up tube is exposed to constant light and the signal output, which comprises the undesired variations is supplied to modulate the period of a multivibrator (see Group XL(c)) the modulated output from which due to a single frame scanning period is fed via a special "gate" circuit (see Group XL(c) ) to the recording head of a magnetic drum store (Fig. 3 not shown). The recording thus made is then employed to effect the necessary compensation for such variation when the pick-up tubs is utilized in normal manner (i.e. producing television signals by scanning a scene) by "reading- off" the signals and after detection in a frequency modulation detection (see Group XL(c) ) employing the signals to vary the gain of a dynamic gain amplifier in the tube output circuit (Fig. 4 not shown). By employing "period" modulation for the recording and then detecting the frequency of the (pulse) carrier when the signals are "read-off" the reciprocal of the signal variation is derived (since period = 1/ frequency) and the derived signal may then be employed to vary the gain of the dynamic amplifier directly. Alternatively, the variations may be recorded as a frequency modulation and the reciprocal derived for the record by detecting the period of the carrier. Instead of employing any modulating system the variation signal may be recorded directly by supplying "samples" of the signals due to each line scan of the pick-up tube successively, to ten separate recording heads spaced along a rotating magnetic recording drum and employing a "reverse" arrangement for "reading-off" the signals the latter then being employed to control the gain of the dynamic amplifier in an inverse manner to effect the required compensation (Fig. 1a and Fig. 2 not shown). When the invention is employed in a three colour, red (R); green (G); blue (B) television transmission system the R and B pick-up tube variations are corrected as described above and, to avoid the effect of noise in the G channel the output of the G pick-up tube is not compensated but the undesired variations in the latter, derived as described above, are utilized to further vary the output of the R and B tubes. Alternatively, the variation signals of the R and B pick-up tubes may be derived by exposing these tubes to an image reproduced in response to signals derived from the G tube when the latter is exposed to a constant light image.